1. Field of the Invention
This invention relates to a method of treating low-integrity dry-formed nonwoven fibrous webs, and more specifically to a method that decorates and enhances the structural integrity of such webs.
2. Description of the Prior Art
Dry-formed, nonwoven fibrous webs for replacing conventional textile fabrics are known in the prior art. Many of these webs are absorbent, strong, abrasion resistant, soft and flexible, i.e. conformable. Such webs can be employed as disposable towels, wipers, or pads, or used as substitutes for conventional textile fabrics in articles such as disposable diapers, sanitary napkins, and so forth.
The webs can be composed entirely of papermaking-length fibers (wood pulp, cotton linters, and other fibers having a length less than about 1/4 inch (6.350 millimeters)), textile-length fibers (man-made and natural fibers having a length greater than about 1/4 inch), or various combinations thereof. Wood pulp fibers are highly absorbent, readily available and more economical to use than textile-length fibers. Consequently, webs made entirely of wood pulp fibers or a preponderance thereof are becoming extremely popular, especially when such webs are intended for single or limited use applications.
Dry-formed, nonwoven webs can be made by an air-laid process, a carding process, or other methods that do not use a liquid to deposit the fibers in the web format. When such a web is initially formed it generally is not self-supporting, i.e. it has very low-structural integrity and mechanical strength. The low integrity and strength of the web results from the fact that the web, as initially formed, is held together primarily by cohesive forces provided by the limited frictional engagement and mechanical interlocking of fibers. In this condition the web has very limited applications, and therefore, for many end-use applications, it is necessary to posttreat the web to enhance its structural integrity and strength.
Methods of posttreating dry-formed, nonwoven fibrous webs have been suggested in the prior art. One such method is disclosed in copending U.S. patent application Ser. No. 569,232, filed Apr. 16, 1975, and assigned to Scott Paper Company. In accordance with that method water is applied to the initially formed web, and the wetted web is then sequentially conveyed through an embossing station, an adhesive application station and a drying and curing station to complete the formation of the nonwoven fabric.
The embossing treatment of the web compresses and densifies the web in the areas that are embossed, and the prior wetting of the web insures that the embossed pattern will be retained in said web. The densification of the web substantially increases the frictional engagement and mechanical interlocking among the fibers, and accordingly enhances the structural integrity and mechanical strength of the initially formed web. In addition, the densification operation produces densified regions in the web which generally have a capillary structure that is more favorable for transmitting liquids than the non-densified, or high loft regions.
Binders, such as acrylic latex, are applied to the embossed web at the adhesive application station to stabilize the fibers and add additional strength and structural integrity to the web. The web is then directed through the drying and curing station to complete the formation of the web. In this condition the web possesses substantial surface strength and structural integrity above that of a web which has only been embossed.
In treating a web it is often desirable to decorate the web by printing colors thereon in a selected pattern. One manner of decorating the web in such a selected pattern is to print a color on the embossed regions that is different from the color of the non-embossed regions. A web so decorated has the appearance of having been "valley printed", that is, printed with a different color in the valley or embossed regions that are recessed below the surface of the non-embossed regions.
Examples of prior art methods of printing completely formed and prepared webs in a selected pattern are disclosed in U.S. Pat. Nos. 1,716,237, issued to Molins; 2,128,516, issued to Graham; 2,168,229, issued to MacArthur; 2,667,426, issued to Davis; 2,674,974, issued to Gwinn et al.; and 2,984,342, issued to Smith. These methods are employed in connection with webs having a high degree of structural integrity, and are not at all concerned with handling and decorating low integrity dry-formed webs. Accordingly, there is no need in these prior art methods to provide for stablization of the web in conjunction with the decorating operation.
Prior art methods for imparting a "valley print" effect to a web are disclosed in the above-mentioned Graham and Davis patents, and also in U.S. Pat. No. 2,858,232, issued to Hushebeck, et al.
Graham is primarily directed to the handling and decorating of textile webs which have a high degree of structural integrity and strength as they are directed through the decorating operation. Accordingly, Graham is not at all concerned with providing a web handling and decorating system for use in connection with low integrity, dry-formed nonwoven webs to both stabilize, or strengthen the web, and decorate it.
Davis relates to mechanically working a web to provide raised areas and ground areas. Davis discloses an arrangement for decorating the web by either tip coating the raised areas or coating the ground areas. In accordance with both arrangements, a roll for transferring decorating fluid to the web is positioned downstream from a roll which mechanically works the web, and both of these rolls are backed by a smooth-surfaced roll 18. This is a somewhat complex arrangement which, when employed to coat the ground areas of the web, requires a somewhat complex gearing arrangement among the web-working roll, the web decorating roll and the backing roll to insure that proper registration of all three rolls is established. Moreover, Davis is not concerned with the problem of handling a low-integrity web to both increase its strength and decorate it. This lack of concern is evidenced by the fact that Davis includes web tensioning bars in the apparatus upstream of the treatment station. Employing such tensioning bars in the transfer of a low integrity dry-formed web to a decorating station would tend to destroy the structural integrity of the web.
Hushebeck et al. discloses a process for producing inlay effects in a fabric web. A non-flowing, thin layer of color paste is applied to the outer raised surfaces of a heated embossing roll. The layer of paste is transferred to the faces of the depressions that are formed when the fabric is embossed. The color pigment must be applied in paste form and it must have a viscosity such that it will remain upon the face of the depressions without running. The method of Hushebeck, et al. cannot be effectively used in treating low integrity dry-formed nonwoven fibrous webs because "fiber picking" will readily occur in such webs. Fiber picking refers to the pulling of fibers from the surface of the web that occurs when the disruptive stress in the treating fluid overcomes the cohesive forces which hold the fibers in the web. When treating fluid is applied to the raised surfaces of a decorating roll and these surfaces are employed to engage the valley regions of a low integrity web, fiber picking will readily occur. This tends to disrupt the integrity of the web's surface. If picking continues, fibers will build up on the raised surfaces and distort the image to be printed on the web as well as cause undesirable spots and depressions in the web. From the above discussion it can be seen that a need exist for a simple and reliable method for strengthening and treating low integrity webs. It is to such a method that the present invention is directed.